In recent years, there have been advances in a reduction in the size of connection terminals in component parts (for example, electronic components in liquid crystal products) due to a trend toward miniaturization and higher functions of electronic devices. Thus, in the field of electronics packaging, anisotropic conductive films, which are film-shaped adhesive agents, are widely used as various adhesive agents for electrode connection, the adhesive agents being capable of easily establishing connections between such terminals. For example, they are used for connections between a flexible printed wiring board (flexible printed circuit board (FPC)) provided with metal electrodes composed of gold-plated copper electrodes and a glass board provided with wiring electrodes composed of indium tin oxide (ITO) electrodes or a rigid board (printed circuit board (PCB)) provided with wiring electrodes composed of gold-plated copper electrodes on a rigid base material such as a glass epoxy base material, between an electronic component such as an integrated circuit (IC) chip and a rigid board, between a flexible board and a printed wiring board, and between flexible boards.
The anisotropic conductive film serves as, for example, an adhesive agent containing electrically conductive particles dispersed in an insulating thermosetting resin such as an epoxy resin. The anisotropic conductive film is sandwiched between connection targets and heated and pressurized to bond the connection targets. That is, the resin in the adhesive agent flows by heating and pressurization, so that, for example, a gap between copper electrodes arranged on a flexible printed wiring board and indium tin oxide (ITO) electrodes arranged on a surface of a wiring board is sealed, and electrical connection between the copper electrodes and the ITO electrodes is established through some electrically conductive particles.
Furthermore, the anisotropic conductive film is required to have electric conduction performance that reduces resistance (connection resistance or resistance to conduction) between connected electrodes facing each other in the thickness direction of the anisotropic conductive film, and to have insulation performance that increases resistance (insulation resistance) between adjacent electrodes in the planar direction of the anisotropic conductive film. In general, for example, a film containing a thermosetting resin such as an epoxy resin as a main component, electrically conductive particles composed of gold, silver, zinc, tin, solder, indium, or palladium, and a microencapsulated latent hardener is disclosed as an anisotropic conductive film having such performance.
For the case of producing the adhesive agent for electrode connection, in general, electrically conductive particles are first added to a solution of an insulating thermosetting resin, e.g., an epoxy resin as a main component dissolved in a predetermined solvent, preparing a composite material for an adhesive agent. Next, the composite material is agitated to uniformly disperse the electrically conductive particles. The composite material is applied onto a film that has been subjected to release treatment, dried, and solidified, thereby providing the adhesive agent for electrode connection.
Here, from the viewpoint that operations (hereinafter, referred to as “repair”) can be easily performed, the operations including peeling the connected electrodes from each other without breakage or damage, removing the adhesive agent with, for example, a solvent, and then connecting the electrodes to each other with the adhesive agent again, it is generally known that a polyvinyl butyral resin, which is a thermoplastic resin, is added when the adhesive agent is prepared. The polyvinyl butyral resin is a solid at room temperature. The polyvinyl butyral resin is softened by heating at a glass-transition temperature (e.g., 60° C.) and can easily be processed. Thus, even if a bonded article of, for example, a flexible printed wiring board and a wiring board is a failure component due to displacement, the wiring board and so forth of the bonded article can be peeled off without damage by heating the polyvinyl butyral resin-containing adhesive agent for electrode connection to a glass-transition temperature. The peeled flexible printed wiring board, the wiring board, and so forth can be used again as component parts of electronic devices.
However, the polyvinyl butyral resin is softened in an environment at a temperature of the glass-transition temperature or higher, thus increasing the mobility of impurity ions in the adhesive agent for electrode connection and reducing the insulation of the adhesive agent for electrode connection. In the case where a current flows continuously between electrodes a bonded article of, for example, a flexible printed wiring board and a wiring board in this state, disadvantageously, insulation failure (i.e., electromigration) due to the migration of metal atoms occurs easily. A higher polyvinyl butyral resin content results in a reduction in insulation. A lower polyvinyl butyral resin content results in a reduction in repairability. Thus, it is disadvantageously difficult to strike a balance between improvement in the insulation of the adhesive agent and improvement in repairability.
Furthermore, in the case where heating is performed to bond, for example, a flexible printed wiring board and a wiring board, the polyvinyl butyral resin in the adhesive agent for electrode connection is softened into a rubber state. So, in the case where pressurization to bond, for example, a flexible printed wiring board and a wiring board is released while the polyvinyl butyral resin is in the rubber state, faulty connections occur because of resilience present in the adhesive agent for electrode connection. It is thus necessary to release the pressurization after the polyvinyl butyral resin is changed into a glass state by cooling the heated adhesive agent for electrode connection to a glass-transition temperature or lower. Hence, in the case where a glass-transition temperature is low, a time period for mounting is disadvantageously increased by the time it takes to release pressurization.
An adhesive agent for electrode connection is disclosed, the adhesive agent containing a thermosetting resin as a main component, a latent hardener, a polyvinyl butyral resin, and electrically conductive particles, in which the thermosetting resin contains a naphthalene-type epoxy resin, and a cured material composed of the thermosetting resin has a glass-transition temperature of 90° C. or higher, the glass-transition temperature being measured by dynamic viscoelasticity measurement (dynamic mechanical analysis (DMA)) (for example, see Patent Literature 1). It is described that the adhesive agent for electrode connection can strike a balance between insulation and repairability and that in the case where, for example, a flexible printed wiring board is mounted on a wiring board, it is possible to reduce a time period for mounting. It is believed that the use of the adhesive agent for electrode connection provides an anisotropic conductive film that strikes a balance between insulation and repairability.